The invention is related to the field of digital data transmission lines and related driver circuits.
FIG. 1 depicts a commonly-used digital data transmission scheme. A transmission line 10 interconnects several communicating devices. The transmission line 10 has a characteristic impedance Z, typically on the order of a hundred ohms, and is terminated by a pullup resistor R.sub.t connected to a termination voltage V.sub.t. Each communicating device employs a transmission line driver such as the illustrated driver circuit 12. A data input signal IN is applied to an inverter consisting of transistors Q1 and Q3, and the output of the inverter drives a pulldown transistor Q2. When the signal IN is LOW, transistor Q2 is ON, and it drives the signal OUT appearing on the transmission line 10 LOW. When the signal IN is HIGH, transistor Q2 is OFF. When the driver pulldown transistor of every device is OFF, the signal OUT is driven HIGH by the combination of resistor R.sub.t and termination voltage V.sub.t.
The driver configuration shown in FIG. 1 is referred to as an "open-drain" driver. As noted, it operates to pull the signal OUT LOW only when it is turned ON. When it is OFF, it merely presents an impedance much greater than Z to the transmission line, so that OUT can be passively pulled high by the pullup termination. This common configuration is used for many reasons, one being the relative ease of attaining electrical compatibility between different devices connected to the transmission line. Each device merely has to present a sufficiently high impedance to the transmission line unless it desires to transmit a logic LOW. The Small Computer Systems Interconnect (SCSI) bus is one example of an open-drain, pullup-terminated transmission line.
One potential problem with any transmission line, including that of FIG. 1, is that of excessive electromagnetic radiation (EMR). EMR can induce noise in nearby electronic devices, and if severe can actually cause them to malfunction. Excessive EMR can be caused by an excessively fast switching rate, or slew rate, of the transmission line driver. As a general goal, then, it is desirable to minimize a driver's slew rate in order to minimize EMR However, an excessively low slew rate disadvantageously increases the overall delay of the driver. Accordingly, it is desirable for a transmission line driver to operate with a slew rate that achieves a desired balance between speed and EMR.
Further complicating the design of transmission line drivers is the extent of normal variation in their operating characteristics. There are numerous factors that contribute to the slew rate of a driver, many of them being subject to considerable variation. Some of the more well-known factors affecting the operation of the driver are process, voltage, and temperature, as well as the impedance of the transmission line to which the driver is connected. A driver is typically designed to meet a maximum delay specification under worst-case process, voltage, temperature, and transmission-line-impedance conditions. Unfortunately, the same driver may have an intolerably fast switching rate, or slew rate, under best-case conditions, and for that reason contribute to excessive EMR. Conversely, drivers can readily be designed to have acceptable slew rates under all operating circumstances, but may then suffer from unacceptable delay under worst-case conditions.